The present invention relates generally to pneumatic valves and more particularly to an EGR valve capable of controlling flow rate under the cooperative control of a vacuum source and an electrically responsive device such as a step motor.
Exhaust gas recirculation (EGR valves) have long found application within a pollution control system of an automotive engine wherein a variable percent of the exhaust gases are recirculated into the intake manifold of the engine. Historically such EGR valves were originally totally vacuum responsive wherein a pressure differential was generated across a flexiable diaphragm. Movement of the flexiable diaphragm is used to vary the position of a pintle valve relative to a valve seat to control the percent of exhaust gases permitted to recirculate into the intake manifold. The performance of such vacuum responsive valves is potentially flawed due to its relatively sluggish response, valve spring relation, aging of the diaphragm material and its dependence upon vacuum pressure which is generally derived from the pressure of the intake manifold. Such vacuum pressure can be shown to reduce substantially during wide open throttle operating conditions. To obtain precise vacuum control these vacuum responsive valves preferably operate in conjunction with a vacuum regulating device which controls the magnitude of vacuum pressure communicated thereto. The totally pneumatic EGR valve may be functionally replaced by a valve which employs a plurality of electrically responsive solenoids as shown in my patent application Ser. No. 659,501 filed 10/10/84 entitled "Electro-Mechanically Controlled EGR Valve and Method." Such system is characterized by a rapid and repeatable mode of operation under all vehicle operating conditions. However, exhaust gas flow rate is varied discretely in steps rather than continuously. U.S. Pat. No. 4,412,517 by Kaboshi et al illustrates an idle speed control valve which is used to bypass a percentage of the input air about the intake manifold to control the idle speed of an engine and employs a step motor to control the amount of air flowing into the air bypass passage. A flow control valve is connected directly to a lead screw whose motion moves the flow control valve. The direct connection of the flow control valve to the step motor results in a system which is characterized by a sluggish response and further requires a relatively massive step motor because of the direct connection to the flow control valve. Characteristic disadvantages of step motors are: step overshoot, limited ability to handle high inertia loads, increased positional error due to frictional loads and the inability to follow every pulse command. These disadvantages contribute to significant position errors when the step motor is used in an open loop manner.
It is therefore an object of the present invention to provide a fast, repeatable and accurately positioned EGR valve which is void of the difficiencies in the prior art. A further object of the present invention is to control EGR flow rate under the combined cooperation of a vacuum source and an electrically responsive mechanism such as a step motor. An advantage of the present invention derived from the dual control aspects recited above is the ability to use of an inexpensive, low inertia, rapidly, responding step motor. In addition, as described in greater detail below the invention includes means for periodically reseting the counters or computer or storage registers associated with the step motor to compensate for any of the control signals for which the step motor did not respond.
Accordingly the invention comprises:
A variable flow rate valve comprising; a housing defining a vacuum chamber. A flexible diaphragm is secured to the housing and defines one extreme of the pressure chamber. The diaphragm is movable in one direction in response to decreases in the pressure within the pressure chamber and movable in an opposite direction in response to increases in the pressure within the pressure chamber. The valve further includes means independent of the diaphragm for providing a mechanical position limit or stop for the diaphragm in one direction and means for selectively varying the pressure within the pressure chamber for causing the diaphragm to move in response to the pressure differential thereacross. The valve may further include means for resetting the control electronics associated with the step motor .